Siemens SM2 monitoring system, U3585-J-Z125-8-76 1 manual Background storage, Caching

Glossary

assignment of paging activities to the initiating task

The system initiates all paging I/O operations directly in the SIH state. However, SM2 does not assign all paging I/O operations to the SYSTEM category, but sometimes to the initiating task.

For SM2, the initiating task is that task which caused a page fault during page access.

Two chains have to be distinguished:

1.Only one page is read.

The full firmware duration of the input operation is assigned to the initiating task and its category.

The full software duration of the input operation is assigned to the category of the initiating task.

The task and category counters for the number of paging operations are incre- mented by 1.

2.Only page output occurs.

The full firmware duration of the output operation is assigned to the PGE task, but not to the SYSTEM category (although it is registered system-globally for SUM). The PGE task counter for the number of paging operations is incremented by 1.

background storage

Storage area on peripheral devices which can be accessed by means of virtual addressing. Programs and data are transferred in pages from background storage to main memory before processing and returned (if required) to background storage after processing.

caching

Caching is understood to be the process of buffering data in a fast data medium (the cache) in an attempt to accelerate subsequent inputs and outputs to the same data areas.

Data to be written to or read from disk is buffered in the cache to avoid the longer input/ output times involved in accessing the disk.

If the data to be accessed is in the cache at the time of access, this is referred to as a cache hit, otherwise as a cache miss.

The proportion of hits in the total number of accesses is known as the cache hit rate. The higher the hit rate, the greater the advantage of using the cache. The hit rate which can be achieved depends on a range of factors, such as the locality of the accesses, the size of the cache, the caching method selected (read cache, write cache, read/write cache) and the appropriate selection of files. Monitoring systems such as SM2 can be used to identify files and disks suitable for caching.

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